In the electronic art one of the problems has been the affixing of microscopic size dies by a layer of adhesive onto a substrate such as a printed circuit board. To date, while a costly machine, equipped with magnifying lenses and movable racks for conveying successive substrate workpieces into magnified position, are well known, it has still been necessary for an operator to manually apply a microscopic deposit of the bonding agent precisely on the substrate ready to receive each die. When the coating, usually of a highly viscous epoxy, has been deposited. The die is then manually or automatically deposited on the coating and the printed circuit then is moved to a heated environment for setting of the epoxy.
In various arts, it has been proposed to provide a liquid transfer apparatus having a source of liquid coating, a coating applicator including a plunger movable within a housing and an arm supporting the applicator for movement between the source of liquid coating and a workpiece.
For example, in U.S. Pat. No. 3,034,478 to Schwartz of May 15, 1962, a deposit of liquid alloy in pellet form is picked up in the open end of an axial bore in a cylindrical housing and then ejected by the tip of a plunger, movable in the housing for deposit onto a workpiece.
In U.S. Pat. No. 3,198,169 to Pike of Aug. 3, 1965, a plurality of plungers are moved upwardly through a bath of liquid, to cause each plunger tip to deposit a coating on the under face of a workpiece such as a clock dial.
In U.S. Pat. Nos. 3,332,394 to Cooke of July 25, 1967, 3,706,274 to Coville of Dec. 19, 1972 and Re. 22,118 to Lohrey of June 16, 1942, pivoted, or oscillating, arms are shown which carry applicators, having no movable plungers, from tip contact with a source of liquid to tip contact with a workpiece.
However in none of the above patents is the pressure of application so controlled as to assure a pick up of a precise thickness of liquid film, a precise deposit of a controlled thickness on a substrate and a uniform thickness of liquid even if the liquid supply surface is uneven or the substrate surface is uneven.